Infection Risks of Pandemic H1N1 Flu, Measles during Air Travel

By Philip R. Fischer, MD, DTM&H, Professor of Pediatrics, Department of Pediatric and Adolescent Medicine, Mayo Clinic, Rochester, MN. Dr. Fischer reports no financial relationships in this field of study.

Synopsis: Two recent studies from the Centers for Disease Control and Prevention evaluated the risks for spread of respiratory viruses from travelers on commercial airplanes. There was no evidence of significant spread of pandemic H1N1 influenza from contacts of two index case travelers with H1N1 during four flights of 1½ to 2½ hours duration. Of 952 passengers on longer flights including someone with active measles, nine developed measles; seven of the nine were among the 4% of travelers known not to have a history of measles illness or vaccination.

While confinement in commercial aircraft is associated with spread of infectious microorganisms, the extent of risk with air travel is incompletely studied. Thus, investigators carefully evaluated outcomes in hundreds of potential contacts of infectious passengers on commercial flights.

Early in the 2009 influenza pandemic, two index cases (a ten year old and a 54 year old) flew commercially. For each, fellow passengers on two separate flights were evaluated, and the majority of passengers participated in the study. Flights were of approximately 1½ to 2½ hours in duration. A total of 290 passengers and 22 crew members were studied. Five percent of passengers and no crew members developed symptoms of an acute respiratory illness during the week after the studied flights, and 2% developed actual influenza-like illness. Neither seating proximity to nor close interactions with the index cases were associated with the development of symptoms. The post-flight illnesses were thought to represent “background noise” rather than evidence of actual in-flight spread of H1N1 influenza.

In a separate study, 74 individuals with active measles were identified after participating in 108 commercial air flights (with a median flight duration of seven hours). Data were evaluated for 79% of the 3399 (non-companion) passenger contacts with these case travelers. Nine secondary cases of measles were identified. Seven of the nine individuals who developed measles after the in-flight exposure were unimmunized with no history of natural measles, one had reported being vaccinated, and one was uncertain about past measles experience. There was a greater risk of getting measles when there was more than one measles-infected passenger on the flight, but the seating distance from the infected travelers was not associated with additional risk of infection.

Commentary

In-flight risks of infection are documented and often discussed. While there has been media coverage of the potential spread of tuberculosis in airplanes, a study of 4550 in-flight contacts of 131 index cases of tuberculosis found no evidence of on-board transmission of tuberculosis.1 Further, a careful review of 13 studies found only two with strong evidence of actual spread of tuberculosis during flight.2 While the risk of transmission of tuberculosis on airplanes is real, it is likely not as great as was previously feared.

Measles, however, seems to be more readily transmitted in airplanes. A previous CDC report in 2011 noted eight secondary cases of measles after shared flights with three patients with measles. Those flights were of 9 ½ and 4 hour durations.3 In another report, three secondary cases of measles seemed to develop following a 4½ hour flight with an index patient; two of the secondary cases were in immunized travelers sitting eight rows behind the index case, and the other apparently picked up measles from between-flight airport contact.4

Similarly, air travel-associated influenza has previously been reported. Mathematical modeling predicted risk of geographical spread of influenza.5 In another review of in-flight influenza transmission, a risk of about 3.5% was identified for people sitting within two rows of the infected subjects.6 This risk is real but relatively low and seems consistent with the study reviewed above.

How is air quality maintained during flight? Filtration systems allow mixing of approximately half outside air with half recycled filtered cabin air7 with the cabin air being recycled every two to three minutes. Filters typically remove infectious agents down to 0.3 micrometers in size, and there is some evidence suggesting that aircraft cabin air is cleaner than the air usually breathed during regular daily activities.8

The field of travel medicine has grown from a foundation in infectious diseases and tropical medicine and now has multiple travel-specific journals, societies, and certifying exams. At the May 2013 13th Conference of the International Society of Travel Medicine in Maastricht, Netherlands, there was widespread realization that much of the current practice of travel medicine relates to non-communicable diseases. Indeed, there was vigorous discussion about non-infectious topics such as deep vein thrombosis and travel during pregnancy and infancy. Still, however, travelers are concerned about getting infected during their trips, and a novel influenza virus and a new coronavirus have emerged this year and started to spread through the world.

So, what can travelers do to decrease their risk of getting or spreading infections during air travel? They should make sure that they are upto-date even on routine immunizations to cover contagious agents like influenza and measles viruses. They should cover their mouths and noses when coughing, and they should sanitize their hands after touching people and shared objects during travel. Reasonable efforts won’t remove all infectious risks of travel, but they will help facilitate safe and healthy excursions around the planet.